2 1 6 HUMAN PHYSIOLOGY. 



5. Internal granular layer. 



6. Molecular layer. 



7. External granular layer. 



8. Membrana limitans externa. 



9. Jacobson's membrane, or layer of rods and cones. 

 10. The layer of pigment cells. 



The most important of these, however, is the layer of rods and cones in the 

 external portion of the retina. The rods are straight, elongated cylinders 

 extending through the entire thickness of Jacobson's membrane. They con- 

 sist of an external portion, which is clear, homogeneous, and highly refract- 

 ing, and of an internal portion, which is slightly granular and less refractive; 

 the outer end of each rod is in direct contact with the pigment epithelium 

 lining the choroid, while the inner end, tapering to a fine thread, pierces the 

 external limiting membrane and passes into the external granular layer. The 

 cones consist also of two portions, the inner of which is somewhat thicker 

 than the rod and rests upon the limiting membrane; the outer portion tapers 

 to a fine point, which is known as the cone-style. The cones, as a rule, are 

 somewhat shorter than the rods. The proportion of rods to cones varies in 

 different parts of the retina, though there are on an average about fourteen 

 rods to one cone. In the macula lutea, where vision is most acute, the rods 

 are almost entirely absent, cones alone being present. All the retinal elements 

 at this point are changed. The nerve-fiber layer is absent, the axis-cylinders 

 radiating in such a manner as to leave the spot free from their covering. The 

 remaining layers are all thinned and the stroma is reduced to a minimum. 

 The optic nerve, after passing forward from the brain, penetrates in succession 

 the sclerotic, choroid, and retina; the nerve-fibers then spread out over the 

 anterior surface of the retina and become connected with the large gangli- 

 onic cells, the third layer of the retina. 



The number of optic nerve-fibers in the retina is estimated to be about 

 800,000, and for each fiber there are about seven cones, one hundred rods, and 

 seven pigment cells. The points of the rods and cones are directed toward the 

 choroid, or away from the entering light, and dip into the pigment layer. 

 They, with the pigment layer, are the intermediating elements in the change 

 of the ethereal vibrations into nerve force; out of these nerve vibrations the 

 brain fashions the sensations of light, form, and color. 



The vitreous humor, which supports the retina, is the largest of the refracting 

 media; it is globular in form and constitutes about four fifths of the ball, it is 

 hollowed out anteriorly for the reception of the crystalline lens. The outer 

 surface of the vitreous is covered by a delicate, transparent membrane, termed 

 the hyaloid membrane, which serves to maintain its globular form. 



